The Impact of HCl to HF Ratio on Hydrated Silica Formation During the Acidizing of a High Temperature Sandstone Gas Reservoir in Saudi Arabia

Abstract

Since the mid-1990s the use of HCl/HF at a weight ratio of 9 to 1 has been used extensively in field operations to minimize precipitation during sandstone acidizing. Although it was perceived as applying to all secondary reactions, it was originally directed at fluosilicates. Field case histories have been reported to support this recommendation. However, little evidence is available to justify the use of 9 wt% HCl - 1 wt% HF (9–1 mud acid) at the 300°F temperature exhibited in the Jauf reservoir. Thus, a laboratory study incorporating HF systems with various HCl to HF ratios was performed to quantify the amount of precipitation in Jauf reservoir cores at 300°F, and the impact on permeability. The focus of this paper is on the silicon (Si) and aluminum (Al) concentrations in core flow test effluents that were corrected for post-core test precipitation of reaction products in collection tubes. This technique accounts for the Si and Al in the liquid and solid phases of core flow effluents. The literature teaches that the Si/Al molar ratio of a core test effluent can be used as a diagnostic tool to quantify hydrated silica formation. It is assumed that hydrated silica precipitation increases as the Si/Al ratio decreases from the theoretical value as a result of a greater completion of the secondary and tertiary reactions of HF with silt and clays. The results of this study on short Jauf reservoir and Berea cores at 300°F indicate that cores treated with 9 wt% HCl - 1 wt% HF yield a lower Si/Al ratio than observed with 12 wt% HCl - 3 wt% HF. However, cores treated with 4 wt% HCl - 1 wt% HF yield the lowest Si/Al ratio. The Si/Al ratio is not the only diagnostic tool required to determine the amount of hydrated silica precipitation i.e. factors including the mud acid flow pattern (matrix vs. channel) have a significant impact on the Si/Al ratio. Core flow test results are presented for acetic acid preflush followed by various mud acid systems. All cores were stimulated, although hydrated silica formed in all core tests and aluminum fluoride precipitated in core tests using 4–1 mud acid formulations at 300°F. Introduction Sandstone matrix acidizing has been applied in oil, gas and injection wells for decades to remove formation damage in the critical area surrounding the wellbore.1,2 The chemical reactions that occur between HF acid systems and the aluminosilicate minerals in sandstone reservoirs have been investigated in detail. It is obvious that the sandstone matrix process is significantly more complex than carbonate acidizing. Unlike carbonate acidizing where the formation damage is simply bypassed via the creation of wormholes, during sandstone acidizing the damage that is plugging the pores normally must be dissolved. Subsequent reactions must not occur that create formation damage i.e. precipitation of calcium fluoride (CaF2) or potassium fluosilicate.3–5 Formation of insoluble reaction products during carbonate acidizing using hydrochloric acid (HCl) is rare, but it occurs in every sandstone acid treatment incorporating an HF acid system i.e. hydrated silica.6–7 The focus of this paper is on the formation of hydrated silica as a function of the HCl to HF ratio in mud acid systems, and its impact on formation damage. Hydrated silica, Si(OH)4, is formed during the secondary and tertiary reactions of HF with feldspar and clay as shown in Equations 1 and 2, respectively.4–9 Secondary Reaction: Equation 1 Silica gel, aluminum fluoride complexes (AlFx(3-x)+, and potassium (K) cations are formed during the secondary reaction (Equation 1) via the reaction of pentafluosilicic acid with feldspar/clay. This reaction goes to completion in most sandstone acidizing treatments before well flowback occurs.4,6,7 Secondary Reaction:Equation 1 Silica gel, aluminum fluoride complexes (AlFx(3-x)+, and potassium (K) cations are formed during the secondary reaction (Equation 1) via the reaction of pentafluosilicic acid with feldspar/clay. This reaction goes to completion in most sandstone acidizing treatments before well flowback occurs.4,6,7

Comparison of Acidizing and Ultrasonic Waves, and Their Synergetic Effect for the Mitigation of Inorganic Plugs

“…The corresponding original permeabilities of the core samples with the initial gas permeabilities of 30 × 10 –3 μm 2 , 80 × 10 –3 μm 2 , and 150 × 10 –3 μm 2 were 38.3%, 44.5%, and 47.5% as shown in Table . This reduction in inorganic plugs can be attributed to the dissolution of CaCO 3 into CO 2 and CaCl 2 by the application of HCl as shown in reaction …”

GLDA/HF Facilitates High Temperature Acidizing and Coiled Tubing Corrosion Inhibition

“…The past 20 years have yielded a wealth of research on HF acidizing that helps explain problems associated with such treatments (Gdanski 1999;Gdanski and Shuchart 1996;Hartman et al 2006;Li et al 1998;Mahmoud et al 2011;Thomas et al 2002;Zhou and Nasr-El-Din 2013). One of the primary problems associated with using APCA fluids in HF acidizing is eliminating the formation and precipitation of CaF 2 .…”

Applications and Laboratory Studies of a New Class of Silica Polymerization Inhibitors of Bio-renewable Origin

“…In principle, it can also lead to amorphous or colloidal silica precipitation if the pH of the spent fluid was allowed to rapidly increase. Thomas et al (2002) additionally provided data for high-temperature work (300°F), indicating that hydrated silica precipitation can be minimized with the application of 9:1 HCl/HF; but, lower ratios, such as 4:1, cannot (Thomas et al 2002). Geochemical simulation, in conjunction with experimental work, indicates one common conclusion: precipitation of colloidal silica is part of the chemical balance and so far appears to be inevitable (Ying-Hsiao et al 1998;Morgenthaler et al 2008;Gdanski and Schuchart 1997).…”

“…The complicated nature of HF-sandstone acidizing is exemplified by the different theories that have been formulated throughout the last decades to explain the reaction of HF on aluminosilicates (Quinn et al 2000;Ying-Hsiao et al 1998;Morgenthaler et al 2008; Thomas et al 2002). Further complicated by the effect of pore network on reactivity (Morgenthaler et al 2008), sandstone acidizing fluids that have HF include either HCl as the base acid or an organic, formic, acetic, or citric acid, or some chelating agents.…”